Brake Line help

[st3v3o]

I have a 70 C10 short bed. Its kind of a frankenstein truck I lucked out with from South Dakota. I recently put a CPP big brake kit on and now working to replace the lines. Im pretty sure its original lines due to the cracking, clogged and mess its in.

I was tracing the lines and getting distances for steel braided brake lines and came across what I think it some sort of junction/pressure valve. Its located under the radiator, on the inside of the frame. Both lines from the master run to it, along with the rear line and both lines for the front calipers. I've tried to research it, Ive asked other buddies who are more savoy with cars and I'm coming up empty handed. Does anyone know what it is or where I can research it?

Thanks